Mechanically actuated electronically controlled unit injectors (MEUI) have seen great success in compression ignition engines for many years. In recent years, MEUI injectors have acquired additional control capabilities via a first electrical actuator associated with a spill valve and a second electrical actuator associated with a direct operated nozzle check valve. MEUI fuel injectors are actuated via rotation of a cam, which is typically driven via appropriate gear linkage to an engine's crankshaft. Fuel pressure in the fuel injector will generally remain low between injection events. As the cam lobe begins to move the plunger, fuel is initially displaced at low pressure to a drain via the spill valve for recirculation. When it is desired to increase pressure in the fuel injector to injection pressure levels, the first electrical actuator is energized to close the spill valve. When this is done, pressure quickly begins to rise in the fuel injector because the fuel pressurization chamber becomes a closed volume when the spill valve closes. Fuel injection commences by energizing a second electrical actuator to relieve pressure on a closing hydraulic surface associated with the direct operated nozzle check valve. The nozzle check valve can be opened and closed any number of times to create an injection sequence consisting of a plurality of injection events by relieving and then re-applying pressure onto the closing hydraulic surface of the nozzle check valve. These multiple injection sequences have been developed as one strategy for burning the fuel in a manner that reduces the production of undesirable emissions, such as NOx, unburnt hydrocarbons and particulate matter, in order to avoid over reliance on an exhaust aftertreatment system.
One multiple injection sequence that has shown the ability to reduce undesirable emissions includes a relatively large main injection followed closely by a small post injection. Because the nozzle check valve must inherently be briefly closed between the main injection event and the post-injection event, pressure in the fuel injector may surge due to the continued downward motion of the plunger in response to continued cam rotation. Thus, if the dwell between the main injection event and the post-injection event is too long, the increased pressure in the fuel injector will undermine the ability to controllably produce small post injection quantities. In other words, the longer the dwell, the larger the post injection quantity becomes. Thus, the inherent structure and functioning of MEUI injectors makes it difficult to control fuel pressure during an injection sequence because the fuel pressure is primarily dictated by plunger speed (engine speed) and the flow area of the nozzle outlets, if they are open. Again, when the nozzle outlets are closed, the fuel has nowhere to go and pressure surges within the fuel injector. As expected, this pressure surging problem can become more pronounced at higher engine speeds and loads, which may be the operational state at which a closely coupled small post injection is most desirable.
The present disclosure is directed to overcoming one or more of the problems set forth above.